Digital performance monitoring for an optical communications system

ABSTRACT

A digital performance monitoring method and system for an optical communications system utilizes a channel monitor and a digital signal processor (DSP). The channel monitor is designed to monitor a respective channel signal of the optical communications system, and includes: a sample memory adapted to store sample data including a set of sequential N-bit (where N≧1) samples generated by an Analog-to-Digital (A/D) converter at a timing of a predetermined sample clock during a predetermined time interval; and a controller adapted to control storage of the sample data to the sample memory. The digital signal processor (DSP) is designed to calculate at least one performance parameter of the optical communications system based on the sample data.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is the first application filed for the present invention.

MICROFICHE APPENDIX

[0002] Not Applicable.

TECHNICAL FIELD

[0003] The present invention relates to performance monitoring systems,and in particular to a system for digital performance monitoring for anoptical communications system.

BACKGROUND OF THE INVENTION

[0004] Optical communications networks are becoming increasingly popularfor data transmission due to their high bandwidth data transmission.Typically, a digital data stream is encoded (e.g. usingOn-Off-Keying—OOK) to generate sequential symbols that are conveyedthrough a communications channel by a respective optical channel signal.At a receiving end of the communications channel, anOptical-to-Electrical (O/E) converter detects the received opticalchannel signal and generates a corresponding analog electrical channelsignal. The O/E converter is followed by an Analog-to-Digital (A/D)converter, which generates sequential N-bit samples (where N is at leastone, and typically between 4 and 8), each of which is indicative of thedetected power of the received channel signal at a particular instant.As such, the samples generated by the A/D converter reflect the combinedeffects of the encoded symbol values, attenuation, noise and any othersignal distortions affecting the channel signal during transmissionthrough the communications network. Thus, at best, the samples generatedby the A/D converter represent a corrupted version of the originalsymbol values.

[0005] Various known decoding strategies may be used to process the“raw” samples generated by the A/D converter to detect and decode thesymbols conveyed by the channel signal, and thereby recover the originaldigital data stream. For example, a digital equalizer may be used toprocess the samples generated by the A/D converter, in order to reduceinter-symbol interference (ISI). A Forward Error Correction (FEC)circuit may then be used to process the equalizer output to determineand generate the correct value of each bit of the recovered digital datastream.

[0006] As the traffic on fiber optic networks increases, monitoring andmanagement of the networks become increasingly important issues. Tomonitor the network, the spectral characteristics of the compositesignal at particular points in the network must be determined andanalyzed. This information may then be used to optimize the performanceof the network.

[0007] Ideally, performance monitoring of an optical communicationssystem should be based on the analysis of the actual optical signalwaveform, across the entire range of wavelengths of interest. Spectralanalysis of this type can be performed using a variety of known signaland spectrum analysis equipment. For example, optical signal analyzersare known for determining characteristics of an optical signal such as,for example, power level, extinction ratio, eye opening, signal-to-noiseratio, polarization dependent loss (PDL), dispersion etc. In order tomonitor respective channels of a Wavelength Division Multiplexed (WDM)communications system, either multiple signal analyzers can be arrangedin parallel, or a single signal analyzer can be sequentially tuned toreceive each optical channel signal in turn. Optical Spectrum Analyzers(OSAs) can be used to determine average and peak power levels, as afunction of wavelength, across any desired range of wavelengths. Thisdata may be used to monitor distributed gain and Raman scatteringeffects within the wavelength band of interest.

[0008] Due to their cost and complexity, conventional optical signal andspectrum analysis equipment is typically restricted to laboratory use.In order to monitor performance of installed optical communicationssystems, simpler and less expensive monitoring equipment is required.Typically, this simplified equipment relies on a low frequency pilottone (typically on the order of 1 MHz or less) that is imposed on theoptical signal at a transmitter end of an optical fiber. The residualpilot tone can then be detected at a desired monitoring point, andcompared with the known parameters of the original pilot tone toestimate a performance parameter of the optical communications system.Suitable detectors can be installed on each channel of a WDMcommunications system to enable calculation of performance parametervalues across a wavelength band of interest. In some cases, processingof per channel measurements can be used to estimate inter-channeleffects. In some advanced optical communications systems, simplifiedanalog Optical Spectrum Analyzers (OSAs) can be used to measure opticalpower as a function of wavelength. OSAs of this type are typically usedin conjunction with optical amplifiers, in order to facilitate controlof pump laser power levels. Typical optical performance monitoringsystems known in the art are disclosed in co-assigned U.S. Pat. Nos.5,513,024; 5,949,560; 5,999,258; 6,128,111; 6,222,652; and 6,252,692.

[0009] While the above-described systems enable some degree ofperformance monitoring, they tend to suffer a number of disadvantages.In particular, per-channel monitoring systems are typically dependent ona low frequency pilot tone (or dither) having known parameters. Anyerror between the design and actual parameter values of the launchedpilot tone will naturally degrade the accuracy of any performanceparameters calculated at the monitoring point. Additionally, thisapproach assumes that performance parameters calculated on the basis ofthe low frequency pilot tone will be valid for the high-speed datatraffic. Consequently, any frequency-dependent effects cannot bedetected (or compensated) with this arrangement. Finally, the detectorsand signal processors utilized in these monitoring systems are lowfrequency analog devices. This facilitates real-time calculation ofperformance parameters using low cost devices. However, this is obtainedat a cost of versatility, and limits the scope of analysis applied tomeasured optical signal parameters.

[0010] Accordingly, a method and system that enables efficientperformance monitoring of an optical communications system remainshighly desirable.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is to provide a method andsystem that enables efficient performance monitoring of an opticalcommunications system.

[0012] Thus an aspect of the present invention provides method ofmonitoring performance of an optical communications system having a datapath including an Analog-to-Digital (A/D) converter operatively coupledto a data decoder for generating a recovered data stream correspondingto subscriber digital data encoded within a respective channel signal.According to the present invention, the data path is tapped to obtainsample data generated by the A/D converter within a predetermined timeinterval. The sample data comprises sequential N-bit samples (where N≧1)respectively indicative of a detected analog value of the channelsignal. At least one performance parameter of the optical communicationssystem is subsequently calculated based on the sample data.

[0013] A further aspect of the present invention provides a digitalperformance monitoring system for an optical communications system. Theoptical communications system has a data path including anAnalog-to-Digital (A/D) converter operatively coupled to a data decoderfor generating a recovered data stream corresponding to subscriberdigital data encoded within a respective channel signal. The digitalperformance monitoring system comprises a channel monitor and a digitalsignal processor (DSP). The channel monitor is designed to tap the datapath to obtain sample data generated by the A/D converter within apredetermined time interval. The sample data comprises sequential N-bitsamples (where N≧1) respectively indicative of a detected analog valueof the channel signal. The digital signal processor (DSP) is designed tocalculate at least one performance parameter of the opticalcommunications system based on the sample data.

[0014] In some embodiments, the channel monitor may also include a datamemory for storing corrected data bits generated using the sample data,for example by a Forward Error Correction (FEC) circuit. The samples andcorrected data bits are preferably correlated, so that a corrected databit can be associated with each sample used to generate that bit.

[0015] In some embodiments, the storage of samples (and possiblycorrected data bits) by each one of a plurality of parallel channelmonitors is controlled such that the storage operation occurssimultaneously. This arrangement allows highly accurate correlation ofsample data across multiple channels, and thus detection of variouscross-channel effects (such as cross-talk, Raman scattering etc.)

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Further features and advantages of the present invention willbecome apparent from the following detailed description, taken incombination with the appended drawings, in which:

[0017]FIG. 1 is a block diagram schematically illustrating principleelements of a conventional optical communications system;

[0018]FIG. 2 is a block diagram schematically illustrating principleelements of a performance monitoring system in accordance with a firstembodiment of the present invention;

[0019]FIG. 3 is a block diagram schematically illustrating principleelements of a channel monitor usable in the performance monitoringsystem of FIG. 2;

[0020]FIG. 4 is a block diagram schematically illustrating a secondembodiment of a channel monitor; and

[0021]FIGS. 5a-c schematically illustrate separation of an optical powerhistogram into respective histograms for binary “0” and binary “1” bitsof a received optical signal, in accordance with an embodiment of thepresent invention.

[0022] It will be noted that throughout the appended drawings, likefeatures are identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0023] The present invention provides a method and system that enableseffective performance monitoring of an optical communications system.FIG. 1 is a block diagram schematically illustrating principle elementsof a conventional optical communications system in which the presentinvention may be deployed. Exemplary embodiments of the performancemonitoring system in accordance with the present invention areillustrated in FIGS. 2-5.

[0024] As is well known in the art, modern optical communicationssystems typically convey both subscriber data signal traffic andadministrative signaling. The subscriber data signal traffic isgenerally non-repeating, and is typically transported at the highestdata rates supported by the communications system. Administrativesignaling (such as, for example, status messages, channel identifiersand test probes) is typically low speed (e.g. on the order of 1 MHz orless), and may be repetitive. An important characteristic of the presentinvention is that performance monitoring of the communications system isaccomplished by analyzing the subscriber data signal traffic, ratherthan low-rate administrative signaling. Accordingly, in the presentapplication, references to; signal traffic, channels, channel signals,data streams, and the like shall be understood to refer to high-speedsubscriber traffic.

[0025] As shown in FIG. 1, a conventional optical communications system2 includes a node 4 which receives high bandwidth (e.g. on the order of10 Giga-bits per second per channel, or more) optical subscriber signaltraffic through an optical fiber 6. For the purposes of the presentinvention, the node 4 may be provisioned as any network element thatperforms optical-to-electrical conversion of the subscriber signaltraffic received through the optical fiber 6. The communications system2 shown in FIG. 1 is configured to receive Wave Division Multiplexed(WDM) subscriber signal traffic, which is optically de-multiplexed, forexample using a conventional optical demultiplexer 8, to separate eachof the individual optical channel signals 10. Each optical channelsignal is then converted to a corresponding electrical channel signal 12by a conventional optical-to-electrical (O/E) converter 14.

[0026] As is well known in the art, the amplitude of the electricalchannel signal 12 generated by each O/E converter 14 is directlyproportional to the optical power of the respective received opticalchannel signal 10. A high-speed Analog-to-Digital (A/D) converter 16 anda data decoder 18 are then used to detect and decode symbols within theelectrical channel signal 12, and thereby generate a recovered digitaldata stream 20 that accurately reflects an originalsubscriber-originated digital data stream. Thus the A/D converter 16samples the analog electrical channel signal 12 generated by the O/Econverter 14 at a timing of a sample clock having a predeterminedfrequency F_(S). The A/D converter 16 generates sequential N-bit (whereN≧2) samples 22, each of which is indicative of an instantaneous valueof the electrical channel signal 12. Typically, the value of N will bebetween 2 and 8, but may be as low as 1, or higher than 8, if desired.As is well known in the art, the frequency F_(S) of the sample clockmust be at least equal to the bit-rate of the channel signal 10, 12 inorder to enable accurate detection and decoding of each symbol withinthe received electrical channel signal 12, and subsequent generation ofthe recovered digital data signal 20.

[0027] As is well known in the art, in some communications systems, eachchannel signal 10 is composed of a pair of orthogonally polarizedoptical modes. Digital data can be encoded into the channel signal usinga variety of known encoding schemes, such as, for example, DifferentialPhase-Shift Keying (DPSK) and Quadrature Phase-Shift Keying (QPSK). Inorder to properly detect and decode symbols within each channel signal10, the optical modes must be separated and independently detected. Insuch cases, the data path of each channel will normally include a numberof O/E detectors 14 and A/D converters 16, so that each optical mode canbe independently detected and sampled. For simplicity of illustration ofthe present invention, this duplication of the O/E detector 14 and A/Dconverter 16 is not shown in the drawings.

[0028] The data decoder 18 may be implemented in various ways to detectand decode symbols in the electrical channel signal 12. It will beappreciated that the data decoder 18 may be implemented in various wayswell known in the art. In the illustrated embodiments, the data decoder18 employs a digital equalizer 24 and a Forward Error Correction (FEC)circuit 26. Other known decoders may omit the equalizer 24 and/orutilize circuitry other than an FEC circuit 26 for generation of therecovered digital data stream 20. The digital equalizer 24 and FECcircuit 26 operate in a conventional manner to process each successivesample 22 generated by the A/D converter 16 to detect and decode eachsymbol of the electrical channel signal 12. As such, the recovereddigital data stream 20 generated by the FEC circuit 26 will, within theerror correction capabilities of the FEC circuit 26, accurately reflectthe original digital data stream conveyed through the received opticalchannel signal 10.

[0029] As shown in FIG. 2, the performance monitoring system 28 of thepresent invention generally comprises a respective channel monitor 30coupled to each channel of the communications system 2, and a digitalsignal processor 32 (DSP). Each channel monitor 30 is substantiallyidentical, and operates to store at least sample data in the form of aset of sequential samples 22 generated by the channel's A/D converter16. In systems having a more than one A/D converter 16 (e.g. forsampling respective mode signals) the sample data will normally containcorresponding sets of sequential samples 22 from each A/D converter 16The DSP 32 may be co-located with one or more channel monitors (e.g. ona network interface card), or provisioned remote from the channelmonitors (e.g. on a different card, a different shelf, or even adifferent site on the network) as desired. In either case, the DSP 32 iscoupled to each channel monitor via a suitable data bus 34 orcommunications link to facilitate transfer of the sample data from eachchannel monitor 30 to the DSP 32. As will be described in greater detailbelow, the DSP 32 operates under control of suitable software to analyzethe sample data to derive a wide variety of both per-channel andcross-channel signal analysis.

[0030] As may be appreciated, the architecture of the performancemonitoring system 28 is substantially independent of the channel plan ofthe optical communications system 2. In particular, each channel monitor30 is substantially identical, and operates independently of the otherchannel monitors 30. Sample data stored by each channel monitor 30 istransferred to the DSP 32 (e.g. under control of the DSP 32) whichperforms desired signal processing and analysis steps. Consequently,changes in the channel plan can readily be accommodated. The design andimplementation of a suitable DSP 32 and data bus 34 structure capable offacilitating communication between the DSP 32 and a very large number ofchannel monitors 30 is well within the purview of those of ordinaryskill in the art. Because each channel monitor 30 operatesindependently, the performance monitor system 28 is substantiallyinsensitive to the wavelength separation between adjacent channels. Thusit will be appreciated that the present invention can be readilyimplemented on WDM communications systems having a flexible channel planand/or bandwidths. Furthermore, the channels of the WDM communicationssystem may have the same, or different, modulation of each channel.Principal elements and operations of a first embodiment of the channelmonitor 30 will now be described with reference to FIG. 3.

[0031] As shown in FIG. 3, each channel monitor 30 comprises a samplememory 36 (which may be provided as a conventional random access memory—RAM) for storing sample data in the form of a set of sequential samples22 generated by the A/D converter 16. The sample data can then be passedto the DSP 32 for processing, as will be described in greater detailbelow. Thus the channel monitor 30 utilizes the A/D converter 16 whichoperates normally to detect the electrical channel signal 12 at a timingof the sample clock. As such, the sample data stored in the samplememory 36 will directly reflect the state of the received electricalchannel signal 12, and thus the performance of the respective opticalchannel. As an additional benefit, the “dual use” of the A/D converter16 reduces the cost and complexity of implementing the performancemonitoring system 28.

[0032] In general, the sample memory 36 may be of any arbitrary size.Preferably, the sample memory 36 will be sized to store samples 22generated by the A/D converter 16 within a predetermined interval oftime (such as, for example, in the range of a few tens of microsecondsto milliseconds) and/or encompassing a predetermined number of samples22, or a predetermined number of data bits generated by the FEC circuit26. Samples 22 may be stored continuously, so that the sample memory 36always contains the most recently generated samples 22. Preferably,however, the storage operation is controlled by a “write” signal 38generated by the DSP. For example, the channel monitor 30 may beprovided with a controller 40 which is responsive to the “write” signal38 to flush the contents of the sample memory 36, and then store apredetermined number of successive samples 22 generated by the A/Dconverter 16. This arrangement has the advantage that the DSP 32 cansimultaneously control the storage of sample data in every channelmonitor 30 of a multi-channel system. By properly accounting forpropagation delay of the “write” signal 38 between the DSP 32 and eachof the channel monitors 30, it is possible to ensure that the storageoperation is executed substantially simultaneously across all of thechannels of the optical communications system 2. As a result, the sampledata stored by all of the channel monitors 30 a-n will accuratelyrepresent a “snap shot” of the state of the optical communicationssystem 2 during the involved time interval. As may be appreciated, thesimultaneous storage of sample data across all of the, channels of theoptical communications system 2 facilitates correlation of sample datafrom each channel monitor, and thus highly accurate analysis ofcross-channel effects such as, for example, cross-talk between adjacentchannels.

[0033] In general, at least some differences in the timing of thestorage operation within each channel monitor 30 must be expected. Thatis, precise simultaneity across all of the channel monitors 30 will notbe achieved. However, provided that there is at least some overlap inthe timing of the storage operation, then sample data from differentchannel monitors can be correlated (at least to the extent that thedegree of overlap permits) and cross-channel effects analyzed. Theminimum tolerable degree of overlap will generally depend on the minimumamount of signal correlation required to analyze a desired cross-channeleffect.

[0034] The transfer of sample data to the digital signal processor (DSP)32 can conveniently be controlled by a “read” signal 42 generated by theDSP 32. This arrangement enables respective sample data from each ofmultiple channel monitors 30 to be transferred to the DSP 32 for furtheranalysis. Synchronized storage of sample data within the sample memory36 of each channel monitor 30 allows indexing of the samples 22 so thatthe transfer of sample data to the DSP 32 does not need to be real-time.Because each channel monitor 30 stores samples 22 representing asubstantially simultaneous time intervals across all channels of theoptical communications system 2, information can be transferred fromeach channel monitor 30, in turn, without loss of correlation betweensamples 22 stored by each of the channel monitors 30.

[0035] As may be appreciated, the accuracy with which the DSP 32 cancalculate performance parameters of the communications system 2 isdependent on the degree to which the sample data stored by each channelmonitor 30 reflects its respective electrical channel signal 12. This,in turn, will be dependent on the resolution of the A/D converter 16,and the sample frequency F_(S) of the sample clock. Clearly, increasingthe resolution of the A/D converter 16 (e.g. using an 8-bit converter asopposed to a 2 or 4-bit converter) increases precision of each sample22. However, this increased precision is obtained at a cost of increasedexpense and complexity.

[0036] Accurate detection and decoding of symbols within the electricalchannel signal 12 requires that the sample frequency F_(s) of the sampleclock must be at least equal to the data rate of the channel signal 10,12. Increasing F_(S) beyond this data rate increases the degree to whichthe sample data stored by each channel monitor 30 reflects itsrespective electrical channel signal 12, and so enables calculation ofperformance parameters with increased accuracy. A sample frequency F_(S)of at least twice the bandwidth of the channel signal 10, 12 satisfiesNyquist's theorem, with the result that the sample data stored in thesample memory 36 will contain sufficient information to enable the DSP32 to completely reconstruct the electrical channel signal 12 receivedduring the involved time interval. This has the advantage of enablingvery detailed signal analysis to be performed by the DSP 32, but again,at a cost in increased cost and complexity of the A/D converter 16 andsample memory 36 to accommodate the required data processing rate.

[0037]FIG. 4 is a block diagram showing a second embodiment of thechannel monitor 30, which also includes a data memory 44 (which may alsobe provided as a conventional RAM) for storing sequential bits of therecovered digital data stream generated by the decoder circuit 18. Thestorage of both sample data 22 and recovered digital data 20 ispreferable, in that it greatly increases the range of analysis that canbe performed by the DSP 32, without an undue increase in cost orcomplexity. As with the A/D converter 16, the channel monitor 30exploits a “dual use” for the decoder circuit 18 already present as partof the data path of the network node 4, and thereby reduces the cost ofimplementing the performance monitoring system 28.

[0038] In the embodiment of FIG. 4, the sample memory 36 is coupled tothe A/D converter 16 as described above in order to store successiveN-bit samples 32 generated by the A/D converter 16. Similarly, the datamemory 44 is coupled to the output of the FEC circuit 26 in order tostore successive data bits of the recovered digital data stream 20.Because the physical characteristics of the A/D converter 16 and thedecoder circuit 18 are well characterized, it is possible to control thestorage operation such that each data bit saved in the data memory 44 isproperly associated with its corresponding sample 22 (or samples) storedin the sample memory 36. In the illustrated embodiment, thisfunctionality is implemented by means of a synchronization circuit 46which operates on the basis of a trigger signal 48 generated by the FECcircuit 26 (and indicative of the timing of each corrected bit generatedby the FEC circuit 26), in combination with the known propagation delaybetween the A/D converter 16 and the output of the FEC circuit 26.

[0039] In general, the sample and data memories 36 and 44 may be of anyarbitrary size. Preferably, each of these memories 36, 44 will be sizedto store corresponding samples and corrected bits 20 within the involvedinterval of time (such as, for example, in the range of a few tens ofmicroseconds to milliseconds), and/or encompassing a predeterminednumber of samples 22 or corrected data bits generated by the FEC circuit26. Samples 22 and corrected bits 20 may be stored continuously, so thatthe sample and data memories 36 and 44 always contain up-to-dateinformation. Preferably, however, the storage operation is controlled bythe “write” signal 38 generated by the DSP 32, as described above withthe reference to FIG. 3. The channel monitor may be responsive to the“write” signal to flush the contents of each of the sample and datamemories, and then store a predetermined number of corrected data bitsand corresponding samples. Thus, the DSP can simultaneously control thestorage of information in every channel monitor 30 across all of thechannels of the optical communications system 2. As will be appreciated,the simultaneous storage of (correlated) sample and corrected bitswithin each channel of the optical communications system facilitateshighly accurate analysis of cross-channel effects such as, for example,cross talk between adjacent channels.

[0040] The transfer of samples and corrected bits to the digital signalprocessor 32 can conveniently be controlled by a “read” signal 42generated by the DSP 32 as described above with reference to FIG. 3.

[0041] If desired, the channel monitor 30 can also be configured (e.g.by means of a suitable memory and connections) to store and transfervarious operating parameters of the A/D converter 16 and decoder 18 tothe DSP 32 along with the samples and corrected bits. Exemplaryoperating parameters may include: threshold levels and sample clockphase used by the A/D converter 16 to generate the samples; andequalizer settings used to control operation of the digital equalizer24.

[0042] As may be appreciated, the storage and transfer of correlatedsamples 22 and corrected bits 20 for each channel facilitates a widerange of sophisticated signal analysis operations to be performed foreach respective channel. For example, FIG. 5A shows an exemplaryhistogram 50 for on-off-keying (OOK) encoded data, which is derived fromsamples 22 stored in the sample memory 36 of a channel monitor 30. Asmay be seen in FIG. 5A, the histogram 50 shows the clustering of samples22 around a high value representing a binary “1”, and a low valuerepresenting a binary “0”. Sample values within each of these clustersare unambiguous, in that a simple threshold comparison accuratelyreproduces the correct data bits. Samples lying between these twoclusters are ambiguous and tend to generate errored bits that must becorrected by the FEC circuit 26.

[0043] In general, the distance between the respective means of the twoclusters is indicative of the eye opening, while the width of eachcluster (which may be represented by the variance or standard deviation,as desired) is indicative of signal noise. As may be appreciated, thesamples 22 stored in the sample memory 36 of the channel monitor 30readily facilitates generation and analysis of the histogram of FIG. 5A,and therefore calculation of eye opening, signal noise, and otherrelevant characteristics of the respective optical channel. Thisoperation can be performed equally in the embodiments of FIGS. 3 and 4.In the embodiment of FIG. 4, the samples 22 stored in the sample memory36 are also unambiguously correlated to respective correct data bits 20(generated by the FEC circuit 26). In this case, it is possible toseparate the histogram of FIG. 5A into respective histograms 52, 54 forbinary “0” bits and binary “1” bits, as shown in FIGS. 5b and 5 c,respectively.

[0044] As mentioned previously, the A/D converter 16 samples thereceived channel signal at a time of the sample clock. In a simpleembodiment of the invention, the sample clock is generated by aclock-recovery circuit (not shown), and is frequency-locked with thereceived channel signal. The phase of the sample clock is preferablyadjusted so that the timing of each sample 22 coincides with theexpected center of each symbol within the electrical channel signal 12.This arrangement has the advantage that timing each sample 22 tocoincide with each received symbol improves the accuracy of decoding anddata recovery, as well as correlation between samples 22 and correcteddata bits 20.

[0045] As is well known in the art, each sample 22 will normally becorrelated with more than one symbol within the received electricalchannel signal 12, which produces inter-symbol interference (ISI). Thedigital equalizer 24 processes each sample 22 in order to correct theISI, and the processed sample supplied to the FEC circuit 26, whichgenerates the corrected data bits. Both the sample 22 and the correcteddata bits 20 are stored for further processing by the DSP 32, asdescribed above. This arrangement provides an M-to-one (where M≧1)correlation between samples and corrected bits, which facilitates a widerange of per-channel signal analysis by the DSP.

[0046] Alternatively, the sample clock can be a free-running clockhaving a frequency greater than the data rate of the received electricalchannel signal 12. In this case, the A/D converter 16 will generate morethan one sample 22 for each symbol within the received channel signal,and the timing of each sample 22 will, in general, not coincide with thecenter of each symbol. According to Nyquist's theorem, if the frequencyof the sample clock is selected so that the A/D converter 16 samples thereceived channel signal at a sample rate F₅, of at least twice thebandwidth of the electrical channel signal 12, then the samples 22stored in the sample memory will contain sufficient information toenable the DSP 32 to completely reconstruct the electrical channelsignal 12. As in the simplified embodiment described above, the digitalequalizer 24 processes each sample 22 in order to correct ISI, and theprocessed samples supplied to the FEC circuit 26, which generates thecorrected data bit. Both the corrected data bit and the correspondingsamples 22 are stored for further processing by the DSP 32, as describedabove. This latter arrangement implies an M-to-one (where M≧2)correlation between samples 22 and corrected bits stored in the sampleand data memories 36 and 44.

[0047] As may be appreciated, reconstruction of the electrical channelsignal 12 enables a more extensive range of per-channel analysis to beimplemented by the DSP 32. For example, a noise spectrum within therespective channel can be detected and characterized.

[0048] In either of the above embodiments, changes in the electricalchannel signal 12 (e.g. between successive time intervals) can bedetected. This information can be used to detect changes in the state ofthe optical communications system 2 and/or determine the sensitivity ofthe electrical channel signal 12 to changes in control parameters usedfor controlling the optical communications system 2. Both of theseresults can be used to optimize the performance of the opticalcommunications system 2

[0049] Because respective sets of samples 22 are simultaneously storedby each channel monitor 30, the respective channel signals reconstructedby the DSP 32 will be time-correlated. This enables the DSP 32 tocompare the signals within respective different channels to evaluatevarious cross-channel effects. Exemplary analysis that may be performedby the DSP 32 include (but are not limited to):

[0050] a) Linear cross-talk between neighboring channels can be measuredby correlation of the respective stored sample data. This informationcan then be used to evaluate (and/or optimize) performance of theoptical demultiplexer, for example.

[0051] b) A known dither can be impressed on, for example, a selectedoptical channel signal or a pump laser at a point up-stream of theperformance monitoring system 28. The DSP 32 can use the known dither toevaluate Raman scattering effects, and this information may be used tooptimize performance of pump lasers within the optical communicationssystem.

[0052] c) Four-wave mixing cross-talk can be evaluated by correlatingthe product of two (or three) signals (as represented by the sample datastored by a pair of channel monitors 30) with a signal measured by athird (or fourth) channel monitor 30.

[0053] d) The location in time of bit errors and the bit error rate canbe calculated for each channel the FEC circuit. The location and/or rateof errors can be correlated with the presence or strength of certainphenomena, e.g. four wave mixing.

[0054] e) Comparison between the samples and data bits stored by achannel monitor 30 can also be used to identify errored symbols withinthe electrical channel signal 12. These errored symbols can becorrelated, in time, to identify error patterns. These error patternsmay be used to identify defective components or other sources of error.They may also be used to identify effects of dispersion, polarizationmode dispersion (PMD) and/or polarization dependent loss (PDL).

[0055] f) Equalizer settings may be used to determine dispersion,polarization mode dispersion (PMD) and/or polarization dependent loss(PDL).

[0056] g) In systems in which the sample data satisfies Nyquist'stheorem, the reconstructed channel signal can be analyzed to detectphase jitter. Correlation between the detected phase jitter and thereconstructed signal of the same and other channels can be used toidentify self phase modulation (SPM) and cross-phase modulation (XPM),respectively.

[0057] The embodiments of the invention described above are intended tobe exemplary only. The scope of the invention is therefore intended tobe limited solely by the scope of the appended claims.

We claim:
 1. A method of monitoring performance of an opticalcommunications system having a data path including an Analog-to-Digital(A/D) converter operatively coupled to a data decoder for generating arecovered data stream corresponding to subscriber digital data encodedwithin a respective channel signal, the method comprising steps of:tapping the data path to obtain sample data generated by the A/Dconverter within a predetermined time interval, the sample datacomprising sequential N-bit samples (where N≧1) respectively indicativeof a detected analog value of the channel signal; and calculating atleast one performance parameter of the optical communications systembased on the sample data.
 2. A method as claimed in claim 1, wherein thepredetermined time interval corresponds with any one of: a selectednumber of symbols conveyed by the channel signal; a selected number ofsamples generated by the A/D converter; and a selected number of bits ofthe recovered data stream generated by the data decoder.
 3. A method asclaimed in claim 2, wherein respective time intervals in which sampledata is obtained from first and second data paths of the opticalcommunications system at least partially overlap in time.
 4. A method asclaimed in claim 3, wherein the respective time intervals aresubstantially simultaneous.
 5. A method as claimed in claim 3, whereinthe step of calculating at least one performance parameter comprisessteps of: identifying a period during which the respective timeintervals overlap; and correlating respective sample data obtained fromeach data path during the identified overlap period.
 6. A method asclaimed in claim 1, further comprising a step of tapping the data pathto obtain decoded data bits of the recovered data stream generated bythe data decoder using the sample data.
 7. A method as claimed in claim6, further comprising a step of compensating a delay between generationof a sample by the A/D converter and generation of a correspondingdecoded data bit by the data decoder.
 8. A method as claimed in claim 7,wherein the step of calculating at least one performance parametercomprises a step of correlating each decoded data bit with at least onesample of the sample data used to generate the decoded data bit.
 9. Amethod as claimed in claim 8, further comprising a step of identifyingerrored symbols within the channel signal, based on the correlationbetween the decoded data bits and the sample data.
 10. A method asclaimed in claim 1, further comprising a step of controlling a samplerate of the A/D converter such that the sample data satisfies Nyquist'stheorem.
 11. A method as claimed in claim 10, wherein the step ofcalculating at least one performance parameter comprises a step ofreconstructing a portion of the channel signal received by the A/Dconverter during the predetermined time interval, based on the sampledata.
 12. A method as claimed in claim 11, further comprising a step ofcorrelating first and second reconstructed portions of the channelsignal received during respective first and second time intervals.
 13. Adigital performance monitoring system for an optical communicationssystem having a data path including an Analog-to-Digital (A/D) converteroperatively coupled to a data decoder for generating a recovered datastream corresponding to subscriber digital data encoded within arespective channel signal, the digital performance monitoring systemcomprising: a channel monitor adapted to tap the data path to obtainsample data generated by the A/D converter within a predetermined timeinterval, the sample data comprising sequential N-bit samples (whereN≧1) respectively indicative of a detected analog value of the channelsignal; and a signal processor (DSP) adapted to calculate at least oneperformance parameter of the optical communications system based on thesample data.
 14. A system as claimed in claim 13, wherein thepredetermined time interval corresponds with any one of: a selectednumber of symbols conveyed by the channel signal; a selected number ofsamples generated by the A/D converter; and a selected number of bits ofthe recovered data stream generated by the data decoder.
 15. A system asclaimed in claim 14, wherein respective time intervals in which sampledata is obtained from first and second data paths of the opticalcommunications system at least partially overlap in time.
 16. A systemas claimed in claim 15, wherein the respective time intervals aresubstantially simultaneous.
 17. A system as claimed in claim 13, whereinthe channel monitor comprises: a sample memory adapted to receivesuccessive samples from the A/D converter; and a controller adapted tocontrol the sample memory to store samples received during thepredetermined time interval.
 18. A system as claimed in claim 17,wherein the controller is responsive to a “write” signal from signalprocessor to control storage of samples in the sample memory.
 19. Asystem as claimed in claim 17, wherein the controller is responsive to a“read” signal from signal processor to control transmission of storedsample data from the sample memory to the signal processor.
 20. A systemas claimed in claim 17, wherein the channel monitor further comprises adata memory adapted to store decoded data bits of the recovered datastream generated by the data decoder using the sample data.
 21. A systemas claimed in claim 20, wherein the controller is adapted to compensatea delay between generation of a sample by the A/D converter andgeneration of a corresponding corrected data bit by the decoder, wherebyeach stored corrected data bit is correlated with at least one sampleused to generate the corrected data bit.
 22. A system as claimed inclaim 20, wherein the controller is adapted to control storage of thesample data in response to a “write” signal generated by the digitalsignal processor.
 23. A system as claimed in claim 22, furthercomprising a data bus adapted to simultaneously convey the “write”signal from the digital signal processor to each one of a plurality ofparallel channel monitors, such that respective sample data is stored byeach of the channel monitors within respective time intervals that atleast partially overlap in time.